1. Field of the Invention
The present invention relates to a fuel cell including electrolyte electrode assemblies and a pair of separators sandwiching the electrolyte electrode assemblies. Each of the electrolyte electrode assemblies includes an anode, a cathode, and an electrolyte interposed between the anode and the cathode. Each of the separators is a single plate.
2. Description of the Related Art
Typically, a solid oxide fuel cell (SOFC) employs an electrolyte of ion-conductive solid oxide such as stabilized zirconia. The electrolyte is interposed between an anode and a cathode to form an electrolyte electrode assembly (unit cell). The electrolyte electrode assembly is interposed between separators (bipolar plates). In use, a predetermined numbers of the unit cells and the separators are stacked together to form a fuel cell stack.
In the fuel cell, an oxygen-containing gas or air is supplied to the cathode. The oxygen in the oxygen-containing gas is ionized at the interface between the cathode and the electrolyte, and the oxygen ions (O2−) move toward the anode through the electrolyte. A fuel gas such as a hydrogen-containing gas or CO is supplied to the anode. Oxygen ions react with the hydrogen in the hydrogen-containing gas to produce water or react with CO to produce CO2. Electrons released in the reaction flow through an external circuit to the cathode, creating a DC electric energy.
In the fuel cell, various proposals for reducing the thickness of the separator and the number of components of the separator have been made in order to reduce the dimension in the stacking direction of the fuel cell. For example, as shown in FIG. 17, according to the disclosure of Japanese Laid-Open Patent Publication No. 2002-75408, a separator 1 of a fuel cell includes a thin sheet of separator body 2, and a large number of first micro-protrusions 3 formed integrally on one surface of the separator body 2, and a large number of second micro-protrusions 4 formed integrally on the other surface of the separator body 2. The first micro-protrusions 3 form a fuel gas flow field 6 between the separator 1 and a fuel electrode 5, and the second micro-protrusions 4 form an oxygen-containing gas flow field 8 between the separator 1 and an air electrode 7.
Further, Japanese Laid-Open Patent Publication No. 10-326624 discloses a fuel cell including an anode, a cathode, an anode gas flow field, and a cathode gas flow field. The anode gas flow field and the cathode gas flow field are separated by a single plate.
Further, for example, a solid oxide fuel cell disclosed in Japanese Laid-Open Patent Publication No. 11-16581 is known. According to the disclosure of Japanese Laid-Open Patent Publication No. 11-16581, as shown in FIG. 18, a plurality of rib members 3a, 3b are radially formed on both surfaces 2a, 2b of a separator 1a. Further, grooves 4a, 4b having a predetermined depth are formed on both surfaces of the separator 1a. The grooves 4a, 4b extend from the outer end to the center of the separator 1a. A fuel gas supply pipe 5a is inserted into the groove 4a, and an oxygen-containing gas supply pipe 6a is inserted into the groove 4b. Each of the fuel gas supply pipe 5a and the oxygen-containing gas supply pipe 6a has a flat tip end. The fuel gas supply pipe 5a and the oxygen-containing gas supply pipe 6a are almost embedded in the separator 1a. 
However, according to the disclosure of Japanese Laid-Open Patent Publication No. 2002-75408, the fuel gas flow field 6 and the oxygen-containing gas flow field 8 are simply formed on both surfaces of the separator 1, and the supply of fuel gas and the supply of the oxygen-containing gas are performed in a limited manner. That is, the fuel gas is simply supplied from one end of the fuel electrode 5 to the other end of the fuel electrode 5, and the oxygen-containing gas is simply supplied from one end of the air electrode 7 to the other end of the air electrode 7. Therefore, the fuel cell cannot be used in wide applications. Depending on the shapes of the fuel electrode 5 and the air electrode 7, it may not be possible to supply the fuel gas and the oxygen-containing gas uniformly to the entire electrode surfaces, respectively. The same problems occur also in Japanese Laid-Open Patent Publication No. 10-326624.
In Japanese Laid-Open Patent Publication No. 11-16581, the flat fuel gas supply pipe 5a and the flat oxygen-containing gas supply pipe 6a are embedded in the separator 1a. Therefore, the thickness of the fuel cell in the stacking direction is large due to the fuel gas supply pipe 5a and the oxygen-containing gas supply pipe 6a. 